Multilayered thermoplastic laminates formed of ABS, polyvinyl chloride, acrylics, polycarbonates, alloys or mixtures thereof have previously been produced by extruding or calendering these materials into thin sheets which are then stacked and subsequently bonded by a process such as fusion bonding, carried out at an elevated temperature.
One method which has been employed to produce extruded products of the type described above is set forth in U.S. Pat. No. 4,107,251 to Bollen et al. The technique disclosed therein comprises extruding a blend of a polycarbonate polymer, a polyethylene terephthalate and a non-acidic silica filler onto a moving support, such as a casting roll, to form an essentially non-oriented sheet. The sheet may then be collected and thermoformed into a variety of useful shapes. Although these products may be quickly formed and inexpensively produced, they suffer, however, from poor abrasion resistance, poor weatherability and dull finishes caused due to their soft, irregular surfaces.
In a further effort to overcome the disadvantages discussed above, attempts have been made, for example, to co-extrude an acrylic layer over a number of different thermoplastic substrates. Such a process is described in U.S. Pat. No. 4,100,237 to Wiley, wherein a molten stream of ABS is joined to a corresponding stream of polystyrene within a single extruder discharge conduit such that a single laminated stream is produced, having a well-defined interface between the adjoining layers. The combined streams are subsequently passed through a sheet form extrusion die having lips generally aligned with the interface.
In some instances, however, the final (co-extruded) product must be produced from normally incompatible polymers. A number of methods have been utilized by those working in the field to overcome this incompatibility, such as that disclosed in U.S. Pat. No. 4,221,836 to Rutledge et al. This patent relates to a process for forming a composite plastic sheet of a solid hydrocarbon polymer and an ABS polymer which comprises co-extruding the two polymers with an intermediate layer interposed therebetween. This intermediate layer comprises an ABS polymer having a bonding factor and a melt viscosity between that of the surrounding layers. These attributes render the intermediate layer strongly adherent to each adjoining polymer layer.
The above-described lamination process developed by Rutledge et al. suffers, however, from several disadvantages. These include the possible delamination of the finished product during the forming process or upon exposure to water when the finished parts are in service. Further, the resultant products suffer from poor weatherability, as well as poor surface quality due to the appearance of die lines and blisters. Additionally, the products must normally be manufactured in a limited variety of colors due to the effort and expense involved in starting and changing the color of materials utilized in a continuous extrusion production line. Also, the material costs required to carry out this process are high because manufacturers cannot re-process scrap material by reentering it into the production line. These materials must thus be discarded as non-recoverable waste.
Various methods for creating an integrated layer structure have also been advanced in the prior art. One such method is described in U.S. Pat. Nos. 4,254,074 and 4,302,497 to Toyooka et al., directed to a process for the production of a decorated synthetic sheet and to the sheet thus produced. The product is formed of a continuously cast sheet of a polymerized synthetic resin syrup which is integrally joined upon at least one side with a uniformly thick preformed film of polymerized synthetic resin, compatible with the cast syrup. The film and the sheet are thus integrally joined to form a homogeneous product.
A different method is set forth in Toyooka's U.S. Pat. No. 4,415,509. This reference discloses a process for forming an integrated laminate of an acrylic resin film having a high light diffusing ability which incorporates an acrylic plate. The process comprises coating an acrylic resin film onto the molding surface of a casting mold, adding an acrylic monomer or a partially polymerized acrylic product into the mold and then polymerizing the product thus produced. The film thus becomes integrated with and laminated upon the surface of the acrylic resin plate.
A coextrusion process for forming integrally joined laminates is disclosed in U.S. Pat. No. 4,415,519 to Strassel. This process involves the coextrusion of: (1) a polyvinylidene fluoride resin layer, (2) a polyalkyl methacrylate layer and (3) a thermoplastic resin, wherein molten streams of these resins are brought into contact some distance prior to exiting the extrusion apparatus so as to optimize the cohesion among adjacent layers. In this manner, the interfaces between adjacent layers become "blended" together into a homogeneous structure, and are no longer easily separable from one another.
None of these processes have been completely successful, however, in overcoming the disadvantages noted above. Articles produced by these processes thus remain prone to delamination, i.e., separation of their layers, during subsequent forming steps or due to mechanical and/or environmental factors when the articles are used.